Ultrahigh vacuum pump



June 28, 1966 F. A. KNOX ETAL I 3,258,196

ULTRAHIGH VACUUM PUMP Filed NOV. 4, 1965 |56 M56 ATTORNEYS United States Patent O 3,258,196 ULTRAHIGH VACUUM PUMP Frank A. Knox and Gaines W. Monk, Fairfax County, Va., assignors to Mount Vernon Research Company, Alexandria, Va.

Filed Nov. 4, 1963, Ser. No. 321,079 35 Claims. (Cl. 230-101) This invention pertains to the art of ultrahigh vacuum pumping and, more particularly, relates to an ultrahigh vacuum pump which combines cryopumping, oil diffusion pumping, oil diffusion vapor trapping, and internal baking in a single integral unit.

The advent of the space satellite program has resulted in a sharp increase in research activity in ultrahigh vacuum environments. Satellites and other components to be used in space are typically tested in test chamber which simulate conditions in outer space. The pressure within such chambers must approximate the virtual vacuum found in space, and this has created a need for a pump capable of evacuating a test chamber to pressure below 8 torr.

In order to achieve pressures of this magnitude, one must use an evacuation system which does not itself contaminate the chamber being evacuated and which Will continue to pump at these low pressures. The nature of the residual gas in a system at this pressure depends greatly on the type of Walls and their treatment. In a commonly used system, such as one made of stainless steel or glass, a large part of the gaseous material, which is given off by the container and objects within it, is of a condensable nature. We have found that, in most cases, cryopumping means, which removes condensables with surfaces cooled to liquid nitrogen temperatures or lower, will reduce the pressure in the system significantly by acting as a high speed pump.

There are also permanent gases emitted from the Walls and evolved by the decomposition of oils, the vapor pressure of which cannot be reduced to sufficiently 10W levels by liquid nitrogen trapping. These noncondensable gases must be removed from the system, and one method of doing this is by means of an oil diffusion pump. The permanent gases diffuse into the oil diffusion pump and are forced out of the system due to impacts by oil molecules in a directed stream or jet.

The uid or oil which is used in the diffusion pump, however, has a vapor pressure at room temperature above the value which one would like to achieve in a test charnber. The heat used to vaporize the oil also tends to generate volatile products. It is therefore necessary to trap or baflie these vapors so that they cannot enter the test system. This trapping is accomplished by means of liquid nitrogen cooled baffles, and decomposition of the oil is minimized by using low temperature heaters in the oil diffusion pump.

It is `also necessary, as a preliminary step, to bake the ultrahigh vacuum system in order to decrease the rate of outgassing from the chamber walls to a sufficiently low level that available pumps can maintain the desired low pressure. This is usually achieved by putting heaters on the outside of the chamber, insulating it, and heating it under vacuum in order to release the sorbed gases. If `this preliminary baking out is omitted, it is impractical to achieve extremely loW pressures Iwith available pumps.

In the past, it has been the practice to employ separate Patented June 28, 1966 cyropumping, diffusion pumping, and cold trapping units and, as was mentioned above, to bake out the system externally. We `have found, however, that this prior expedient has many drawbacks. In addition to the obvious inconvenience of using separate units, it is difiicult optimally to match the cryopumping and cold trapping units to the characteristics of the diffusion pump employed. Each of the separate units requires separate mounting flanges and sealing gaskets, and the assembled separate units tend to be wasteful of vertical space. Moreover, external baking of the system tends to be inefficient, increasing the pumping burden on the pump units.

It is accordingly the principal object of the invention to provide an improved ultrahigh vacuum pump.

Another object of the invention is the provision of an economical, convenient and vertically compact ultrahigh vacuum pump having a combined cryopumping and cold trapping section optimally matched to the characteristics of a diffusion pumping section.

A more specific object is the provision of an ultrahigh vacuum pump, combining cryopumping, diffusion pumping, cold trapping, and internal baking in a single, integral housing.

An additional object of the invention is the provision of a pump of this type in which the cryopumping means may be conveniently removed from the housing for servlcmg.

A further object of the invention is the provision of a pump of this character in which the cryopumping means does not diminish the efficiency of the diffusion pumping means and in which the cryopumping means comprises a tank for a cryogenic fluid which is spaced from the Walls gf the pump housing whereby the tank is insulated there rom.

A further object of the invention is the provision of improved cryogenic tank and baffle means for trapping condensable gases.

A still further object is the provision of a diffusion pump having improved means preventing migration of oil vapor into a system being evacuated.

Yet another object is the provision of an ultrahigh vacuum pump having internal baking means.

Still another object is the provision of feedthrough means through the pump barrel and the tank for gaining access to the interior of the cryopumping section.

It is `a further object of the invention to provide improved, nondraining means for filling the cryogenic tank with a cryogenic fluid, such as liquid nitrogen, in a liquid phase and for preventing the cryogenic fluid remaining in the fill pipe from freezing the supply valve after the tank is substantially filled. A related object is the provision of improved means for removing the cryogenic fluid in a gaseous phase from the tank.

Other objects include the provision of an improved diffusion pump having a self-aligning jet stack assembly, the provision of means for quickly cooling the boiler of a diffusion pump upon shut-down of the ultrahigh vacuum pump for servicing, and the provision of an improved anti-bumping honeycomb structure.

Brieliy, the invention ycontemplates the inclusion of a combined cryopumping and cold trap unit and a diffusion pump within a com-mon, integral pump housing or Vbarrel having an upper housing section and a lower housing section of smaller diameter connected together by a shoulder section. The cryopumping and cold trap unit comprises a substantially toroidal tank for a cyro genie fluid, such as liquid nitrogen, located above the shoulder and spaced from the walls of the upper housing section. The tank is filled by means of a non-draining lill pipe detachably extending through the shoulder and: upwardly into the tank through its bottom wall to a point adjacent the top wall of the tank and then bending downwardly to a discharge opening adjacent its bottom wall. The cryogenic lluid may be removed from the tank in a gaseous phase by means of an exhaust pipe having an exhaust opening adjacent the top wall of the tank and extending downwardly through the tank through its bottom wall and detachably through the shoulder. The inner wall of the tank supports `a baille system which comprises a pair of spaced annular inwardly extending flanges and a centrally located baille supported on a pipe, extending lacross the axial opening of the tank and conducting cryogenic lluid thereto from the tank. In order to bake out the system, a heater is wound on the inner wall of the tank between the annular flanges of the baille. The diffusion pump is housed within the lower housing section and comprises a self-aligning jet stack assembly. A baille cap substantially larger than the upper jet cap of the diffusion pump is positioned immediately thereabove to inhibit migration of oil vapor into the system being evacuated. In order to facilitate quick shut-down of the system for servicing, a quick cool plate is located immediately below the heater of the diffusion pump boiler. An anti-bumping honeycomb structure comprising three superposed, staggered honeycombs is placed in the boiler.

The foregoing and other objects, advantages, and features of the invention and the manner in which the same are accomplished will become more readily apparent from consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate a preferred and exemplary embodiment, and wherein:

FIGURE l is a front elevation, partially in section, of a pump unit of the invention;

FIGURE 2 is a plan view, partially in section along the lines 2-2 of FIGURE 1;

FIGURE 3 is a fragmentary elevation view, partially in section, and somewhat distorted in scale, showing the manner in which a cryogenic fluid is fed to a tank of the invention; and

FIGURE 4 is a fragmentary elevation view, partially in section, showing a detail of FIGURE l enlarged in scale.

Referring now to the drawings, and especially to FIG- URE 1, it will be seen that an ultrahigh vacuum pump of the invention comprises an upper cryopumping and cold trap section and a lower diffusion pumping section 12. As shown in this figure, the two sect1ons are enclosed in a common pump housing or barrel 14. The pump barrel has an upper housing section 16 and a lower housing section 18 of smaller diameter. The two housing sections are connected by means of a substantially horizontal shoulder 20.

The cryopumping and cold trap section 10 comprises a substantially toroidal -tank 22 having an outer cylindri- -cal wall 24, an inner cylindrical wall 26, a ltop wall 28, and a bottom wall 30. It will be noted that the outer' cylindrical Wall 24 is of smaller diameter than housing section 16 and is thereby spaced therefrom by means of a space 32 which serves, by virtue `of the low pressure within the system, as an insulating space for the tank. The inner cylindrical wall 26 defines an axial passageway 33 for gases being evacuated from a test chamber or system 34 coupled to the pump.

In order to facilitate such coupling to the test chamber 34, the upper housing section 16 is provided with an external mounting ring 36 welded about its open upper end 37. Mounting ring 36 has an outer annular portion 38 extending above end 37 to define an inner mounting shoulder 4t) in the plane of end 37. The test chamber 34 to be evacuated is Seated on mounting shoulder 40 within outer annular portion 38. Suitable bolt holes 42 extend through ring 36 (see FIGURE 2) for bolting it to a mounting flange 44 of test chamber 34. Metallic gaskets (not shown) are provided between ring 36 and flange 44 for sealing the system.

Tank 22 is filled with a cryogenic fluid 48, which is preferably liquid nitrogen, by means of a fill pipe S0 coupled to a supply of the fluid through a supply valve (not shown). In order to prevent liquid nitrogen within tube 50 and tank 22 from freezing the supply valve, fill pipe 50 is arranged as shown in FIGURE 3. As will be seen from this figure, lill pipe 5l) extends upwardly through a shoulder wall portion 52 which closes a tube 54 depending downwardly from shoulder 20. The ll pipe then extends upwardly through bottom wall 30 of tank 22 to a point just below the top wall 28 of the tank where it is provided with a U-bend 56 and thereafter extends downwardly to a point adjacent, but above, the bottom wall 30, where it is provided with a discharge opening 58. It will be noted that the lill pipe 50 is detachably connected to wall portion 52 by means of a nut 60 threaded on a threaded sleeve 62 tted over lill pipe 50. The upper end of threaded sleeve 62 is provided with a flange 64 which is vacuum welded to illl pipe 5l) and which bears downwardly against a metallic gasket 66 to seal the opening through wall portion 52.

When it is desired to lill tank 22, the supply valve is opened and the cryogenic fluid 46 is fed upwardly through lill pipe 50 and discharged into tank 22 from discharge opening 58. When the liquid nitrogen reaches a desired level 68 within tank 22, the supply valve is closed. Since the portion of the ll pipe 50 between nut 60 .and the supply valve will be exposed to substantially ambient conditions, `the liquid nitrogen therein will evaporate and cause the liquid nitrogen in the pipe between nut 60 and bend S6 to be blown around bend 56. The gaseous nitrogen in pipe 50 between the valve and bend 56 will then serve as a gas lock and will keep the supply valve relatively warm and unfrozen. It will `also be observed that the lill pipe 50 is so arranged that it ordinarily cannot drain the cryogenic fluid from the tank.

The manner in which the liquid nitrogen is removed 4from the tank 22 will be apparent from a consideration of FIGURE 1. As there shown, exhaust pipe 70 extends upwardly through a shoulder wall portion 72, which closes a tube 74 depending downwardly from shoulder 20, and extends upwardly through tube 74, through bottom wall 30 of tank 22, and upwardly through tank 22 to a point, adjacent, but below, top wall 28 of tank 22 and above the level 68 of the liquid nitrogen within the tank. An exhaust opening 76 is provided at the upper end of exhaust pipe 7). Thus, exhaust pipe 70 will remain lled with gaseous nitrogen which will serve as a gas lock between opening 76 and an exhaust valve (not shown), keeping the exhaust valve relatively warm and unfrozen. When it is desired to remove the cryogenic fluid from the tank 22, the gaseous nitrogen above level 63 is pumped out of exhaust pipe 70, as by forcing a pumping gas through the supply valve, thereby lowering level 63 until all of the nitrogen within tank 22 is removed. Alternatively, the liquid nitrogen may be dumped from tank 22 in liquid form by closing the exhaust valve and opening the supply valve. As the pressure of the gaseous nitrogen above level 68 increases (due to the closed exhaust valve), the liquid nitrogen will be forced around bend 56 and through the supply valve.

As in the case of lill tube Sil, exhaust pipe 70 is detachably connected to the shoulder wall portion 72 by means of nut 7S. It will be understood that a threaded sleeve having a flange pressing against a metal gasket, as shown in FIGURE 3, is employed in making the detach-able connection of exhaust pipe 70 to the lower wall portion 72. However, for the sake of clarity, these parts have been omitted from FIGURE 1.

Tank 22 is mounted in spaced relation from the Walls of housing 14 by means of a mounting block 79 of stainless steel between bottom wall 30 and shoulder 20. This mounting block 79, fill pipe 50, and exhaust pipe 70 serve to support tank 22 with the outer wall 24 spaced from housing section 16 and the lower wall 30 spaced from the shoulder 20. In this way, the substantial vacuum within the spaces serve to insulate the tank and thereby reduce heat flow and heat lloss therefrom. While more than one mounting block 79 may be employed, it is preferred to employ only one to minimize heat flow from the tank to shoulder 20.

An important aspect of cryopumping and cold trap section is a baffle system generally designated by reference numeral 80. The bame system comprises a pair of spaced annular baffle flanges 82 and 84 extending inwardly rfrom inner wall 26 of tank 22, and defining a pair of aligned openings 86 and 88 coaxial with inner cylindrical wall 26 of tank 22 and gas passageway 33. A pair of substantially semicircular bafe plates 90 and 92 are located midway between baffle anges 82 and 84, extend in a plane normal to gas passageway 33, and are mounted on a pipe 94 which extends diametrically across gas passageway 33 between opposed openings 96 and 98 through inner wall 26 (see FIGURE 2). The pipe 94 thus serves as a support for baffle plates 92 and 90 and as means for conducting liquid nitrogen from tank 22 to the plates to maintain them substantially at he temperature of liquid nitrogen.

In order to bake out the ultrahigh vacuum pump and cause out-gassing of trapped gases prior to operation of the pump, a helical electric heater 100 is mounted upon the inside surface of inner wall 26 of tank 22 between baffle flanges 82 and 84. (Only a portion of heater 100 is shown in FIGURE l, the remainder of the heater having been omitted for clarity.) The heater 100 is preferably of the type comprising a heater wire encased within a sheath of stainless steel or other heat resistant alloy and is energized by means of an electric power lead 102. In order to provide access for lead 102, a feedthrough opening 104 extends through tank 22 between the outer cylindrical wall 24 and the inner cylindrical wall 26 in alignment with a feedthrough port 106 provided through pump housing section 16. A cylindrical boss 108, about port 106 and integral with pump housing section 16, extends outwardly therefrom and is sealed at its open end by means of a closure plate 110 which is bolted thereto by means of bolts 112. A suitable metallic sealing gasket (not shown) is provided between plate 110 and the end of boss 108, and the heater 102 is fed through plate 110 by means of a sealed feedthrough coupling 114. An additional feedthrough port 116 may be provided through tank 22 in alignment with an additional feedthrough port 118 through upper housing section 16. As with feedthrough port 106, feedthrough port 118 is surrounded by a cylindrical boss 120, extending outwardly from upper housing section 16, and is sealed by means of a closure plate 122 bolted to the end of boss 120 by means of suitable bolts 124 to compress a metallic gasket (not shown) therebetween. Appropriate apparatus, such as a vacuum gage, may be coupled to the end of cylindrical boss 120, as desired.

Turning now to diffusion pump section 12, it will be observed that lower housing section 18 encloses a jet stack assembly 125 of a diffusion pump. The lower end of housing section 18 serves as a boiler 126 for pumping oil 12.7. The oil is heated by means of a substantially annular electrical heater 128 mounted (by means to be described hereinafter) contiguous to the outer surface of the bottom wall 130 of housing 14. As shown, heater 128 is suitably energized through a power line 132.

Since it is sometimes necessary to shut-down the pump for servicing, it is desirable to quickly cool the boiler. This is accomplished by means of a quick cool plate 134 which is mounted below and contiguous to heater 128. In order to quickly cool the quick cool plate 134, a spiral water cooling pipe 136 is mounted on the lower surface of plate 134. The heater plate 128 and quick cool plate 134 are enclosed by a reflector housing 138. As shown most clearly in FIGURE 4, quick cool plate 134 is clamped against heater 128 and heater 128 against bottom wall of housing 14. For this purpose, a nut 140 is welded to the center of bottom wall 130; and a bolt 142 is threaded therein. The bolt head 144 of bolt 142 bears against the lower surface of quick cool plate 134 to clamp it against heater 128 and heater 128 against lower wall 130. In order to maintain reflector housing 138 in position, bolt head 144 is provided with a threaded opening 146 and receives a bolt 148 which extends through an opening 149 in reflector housing 138 and is provided with a bolt head 150 bearing against the lower surface of reflector housing 138.

Since the oil in boiler 126 may become subject to violent boiling and cause oil to splash into the jet stack assembly 125, We mount an anti-bumping honeycomb or grid structure 151 on the bottom wall 130 of housing 14 as is customary in the art. We have found, however, that 'the eciency of the honeycomb structure is greatly enhanced by providing the honeycomb structure 151 as three superposed honeycombs 152, 153 and 154 (see FIGURE 4) which are staggered with the open-ended cells of adjacent honeycombs in misalignment. This serves to break up vertical flow of bubbles and prevent the ow of large bubbles. While honeycombs comprising a plurality of hexagonal cells are preferred, it is to be understood that the cells may be of any polygonal form and may, for example, be square. The honeycomb structure 151 also inhibits lateral flow of oil; this promotes fractionating of the oil, directing the most volatile oils to the outermost jet stack and the least volatile to the innermost stack, as is known in the art.

The jet stack assembly 125 is supported by a mounting ring 155 placed Within boiler section 126 and having spaced feet 156 resting on lower wall 130. As will be explained more fully hereinafter, oil vapor from the vapor jets yof the diffusion pump will condense on the inner surface of housing section 18. In order to permit this condensed oil to return freely to boiler 126, mounting ring 155 is spaced inwardly a small distance from the wall of lower housing section 18 and is provided with openings 158 between feet 156. A baseplate 160 is welded within the upper end of mounting ring 155, is spaced slightly above honeycomb 154, and is provided with a plurality `of perforations 161 (see FIGURE 4), permitting vapor produced in the boiler to ow upwardly into the jet stack assembly 124. An outer jet stack 162 of assembly 125 is set in a groove 164 in baseplate and extends upwardly therefrom. A jet cap 166 is suitably mounted by means of spaced mounting struts (not shown) upon the upper end of jet stack 162` and is provided with an outer downwardly extending jet deflecting flange 168 and an inner flange 170 defining a central opening 172 and a mounting shoulder 174. A second jet stack 176 rests on mounting shoulder 174 and extends upwardly therefrom. A second jet cap 178 is mounted by suitable mounting struts (not shown) upon the upper end of jet stack 176 and is provided with an outer jet deflecting flange 180 and an inner mounting flange 182 which defines a downwardly facing shoulder 184, an upwardly facing shoulder 186, and a central opening 188. The downwardly facing shoulder 184 bears against the upper end of a jet stack section which fits snugly within jet cap 178, while an upper jet stack section 192 is mounted on the upper shoulder 186, fitting snugly within jet cap 178. It will be observed that the lower end of jet stack section 190 is set in a groove 191 in baseplate 160. The upper end of jet stack section 192 supports a third jet cap 194 by means of supporting struts (not shown). Jet cap 194 is provided with an outer downwardly extending jet defiecting ange 196 and a central opening 198. A tie rod 200 extends through central opening 198 and downwardly through jet stack section 192, opening 188, jet stack section 190, and an opening 202 through baseplate 160; a nut 204, bearing against the upper surface of jet cap 194, and a nut 206, bearing against the lower surface of baseplate 160, are threaded on opposite ends of tie rod 200. Jet stacks 162 and 176, jet stack sections 190 and 192, and jet caps 166, 178 and 194 are thus brought into proper alignment; accordingly, jet stack assembly 125 may be considered as self-aligning.

Since oil vapor migrating upwardly from the oil jets of the diffusion pump would contaminate the chamber 34 being evacuated, a baffle cap 208 of substantially 4greater diameter than upper jet cap 194 is mounted irnmediately thereabove on a spider 210 supported on shoulder 20. The diameter of baie cap 208 is selected to emphasize reduction of upward migration of oil vapor. It will be understood, however, that it cannot be made too large without unduly reducing the pumping speed of the diffusion pump. The diameter of baffie cap 208 is thus selected as a comprise between maximum vapor bathing and adequate pump speed.

The operation of the diffusion pump will be understood by reference to the arrows in FIGURE l which show the flow of vapor. Thus, vapor 211 formed in the boiler 126 fiows upwardly therefrom, As explained above, the oil is fractionated. That is to say, oil in the outer peripheral portions of the boiler 126 are cooler and vaporization products therefrom flow upwardly to the rst jet cap 166 and are downwardly directed therefrom at 212. Vapor products from portions closer to the center of boiler 126 are boiled off at a higher temperature and flow upwardly to the second jet cap 1'73 where they are downwardly directed therefrom to form a second jet 214. Finally, the least volatile `oil fractions are evaporated at the center of the boiler where it is hottest and extend upwardly through openings 161 in baseplate 160 into the central jet stack formed by jet stack sections 190 and 192 to form the upper jet 216. It will be understood that the diffusion pump operates in the usual way and that the jets provide oil vapor molecules which impinge against gas molecules being evacuated from the system directing -them downwardly to a discharge opening 213. A water cooling coil 220 is wound about the outside of pump housing section 18 to cool the housing wall and condense the oil vapor which then return to the boiler 216, as explained above.

Gases evacuated through discharge opening 218 are fed through discharge pipe 222 which is coupled at its end 224 to a suitable forepump (not shown). If desired, pipe 222 is provided with an integral branch pipe 226 which may serve as a gage connection.

While it is believed that the operation of the ultrahigh vacuum pump of the invention will be apparent from the above description, it will be now described in more detail. The test chamber or system 34 to be evacuated is seated on mounting shoulder 40, and the mounting ring 38 is bolted to the mounting flange 44 of the test chamber. The forepump coupled to discharge pipe 222 is turned on to provide a low backing pressure for the ultrahigh vacuum pump. The heater 100 is then energized to bake out the trap` section 10, driving off sorbed gases and vapors, and to radiate power into test chamber 34, jet stack assembly 125, and pump housing 14. The gases are released from the heated surfaces and are pumped out of the system. After this preliminary baking, the diffusion -pump section 12 is turned on by energizing heater 128 and feeding cooling water to coil 220 and the baking process is continued. This allows removal of gases from test chamber 34 to a very low pressure due to the high speed of diffusion pump section 12.

The heater 100 is then de-energized, and the supply valve is opened to fill tank 22 through fill pipe 50 with liquid nitrogen to level 68. The supply valve is then closed. It will be understood, that the evaporation of liquid nitrogen in pipe 50 below nut 60 serves to blow any liquid nitrogen remaining in fill pipe 50 below U-bend 56 around the bend, thereby preventing freezing of the Valve in its closed position. Condensable gases then impinge upon the walls of tank 22 and upon baffle plates 32, 84, and 92, which are now chilled to a temperature of about 323 F., and are thus removed from the system. The condensed gases will remain on these chilled surfaces until the next time the trap section is warmed; they will then evaporate and be removed from the pump. Non-condensable gases will flow downwardly through cryopumping section 10 and diffuse into the diffusion pumping jets 212, 2114 and 216. The non-condensable gases are thus driven downwardly to discharge opening 218 and out discharge pipe 222 to the forepump. Should any of the oil vapor from the diffusion jets 212, 214 and 216 have a tendency to migrate upwardly, it is either downwardly deflected at 228 by means of the baffle cap 208, or trapped in the cryopumping section 10 by coming int-o engagement with the chilled baffle flanges 84 or 82, baffle plates 90 and 92, or the walls of tank 22. In this way, oil vapor is prevented from migrating upwardly into the system 34 being evacuated.

It will be observed, that the diameter of the inner wall 26 of the tank 22 is substantially equal to the diameter of lower housing section 18. The tank 22 thus does not significantly reduce the size of gas passageway 33 leading to the diffusion pump section 12 and therefore does not tend to reduce significantly the efficiency of the diffusion pump. In this way, an optimal match between sections 10 and 12 may be obtained.

All openings to housing 14 are sealed by means of metallic gaskets which will withstand the baking temperature provided by heater 100. Th'e gaskets are preferably made of relatively soft metal, such as aluminum, silver, or gold. The jet stacks and jet caps are preferably made of aluminum, while the remaining parts of the pump are constructed of stainless steel.

In order to enhance servicing of the pump, the tank 22 is removed by loosening nuts 62 and 78. Tank 22 is then simply lifted upwardly through upper opening 37 at the upper end of upper housing section 16, thereby exposing diffusion pump section 12.

While a preferred embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. Accordingly, the foregoing embodiment is to be considered illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalency of th'e claims are to be included therein.

The invention claimed is:

1. An ultrahigh vacuum pump for evacuating an ultrahigh vacuum system comprising cryopumping means for removing condensable gases from said system, diffusion pumping means for removing non-condensable gas'es from said system, a single integral housing enclosing said cryopumping means and said diffusion pumping means, said housing having an open end and a substantially closed end, said cryopumping means being located between said diffusion pumping means and said open end, and detachable means including a ll pipe and an exhaust pipe for `said cryopumping means for supporting said cryopumping m'eans in said housing so that said cryopumping means may be readily removed from said housing through said open end to provide access to said diffusion pumping means for servicing.

2. An ultrahigh vacuum pump as recited in claim 1, said open end being at the upper end of said housing,

and means for coupling said system to said open end, said cryopumping means being located above said diffusion pumping means, said diilusion pumping means including means providing an oil diffusion jet, said cryopumping means serving as a trap preventing migration of oil vapor from said jet upwardly into said system.

3. An ultrahigh vacuum pump as recited in claim 2, further comprising a circular baille plate located immediately above said diflusion pumping means, said jet providing means including jet forming means at the upper end of said diflusion pumping means, said baille plate having a diameter substantially greater than said jet forming means to reduce migration of oil vapor from said jet upwardly into said system.

4. An ultrahigh vacuum pump as recited in claim 1, said cryopumping means comprising a tank, means feeding a cryogenic fluid to said tank, and means spacing said tank from the walls of said housing whereby said tank is insulated from said housing.

5. An ultrahigh vacuum pump as recited in claim 4, wherein said cryogenic fluid is liquid nitrogen.

6. An ultrahigh vacuum pump for evacuating an ultrahigh vacuum system comprising cryopumping means for removing condensable gases from said system, diffusion pumping means for removing non-condensable gases from said system, and a single housing enclosing said cryopumping means and said diflusion pumping means, said housing comprising a first section surrounding said cryopumping means, a second section surrounding said diffusion pumping means, said first section being of greater transverse dimension than said second section, and a shoulder connecting said first section to said second section; said cryopumping means comprising a tank located above said shoulder, means extending through said shoulder for filling said tank with a cryogenic fluid, and means extending through said shoulder for removing said cryogenic fluid from said tank.

7. An ultra-high vacuum pump as recited in claim 6, further comprising an opening at the upper end of said first section, said opening having substantially the same transverse dimension as said first section, means for coupling said system to said op'ening, said means for filling said tank comprising a fill pipe, said means for removing said fluid from said tank comprising an exhaust pipe, means for detachably connecting said fill pipe through said shoulder, and means for detachably connecting said exhaust pipe through said shoulder, whereby said tank may be removed from said housing through said opening upon detaching said lill pipe and said exhaust from said shoulder.

8. An ultrahigh vacuum pump as recited in claim 6, said means for filling said tank comprising a lill pipe extending upwardly through said shoulder and said bottom wall to a first point adjacent, but below, said top wall, said fill pipe having a U-bend at said first point and extending downwardly to a second point adjacent, but above, said bott-om wall, said fill pipe having a discharge opening at said second point; and said means for removing said fluid from said tank comprising an exhaust pipe extending upwardly through said shoulder and said bottom wall to a third point adjacent, but below, said top wall, said exhaust pipe having an exhaust opening at said third point.

9. An ultrahigh vacuum pump as recited in claim 6, said tank having an inner wall defining a gas passageway, an outer wall surrounding said inner wall, a top wall and a bottom wall, said outer wall being spaced from the wall of said first housing section and said inner wall having substantially the same transverse dimension as said second housing section.

10. An ultrahigh vacuum pump as recited in claim 9, said tank being substantially toroidal, said housing sections and said inner and outer Walls being substantially cylindrical, said first housing section being of greater diameter than said second housing section, said inner wall having substantially the same diameter as said second section, and said outer wall being of smaller diameter than said first section.

11. An ultrahigh vacuum pump as recited in claim 9, said cryopumping means comprising baille means extending inwardly from and supported by said inner wall.

12. An ultrahigh vacuum pump as recited in claim 11, said baille means comprising a baille plate located within said passageway, and a pipe extending transversely across said passageway and being coupled to said tank through openings in said inner wall, said baille plate being mounted on said pipe.

13. An ultrahigh vacuum pump as recited in claim 9, further comprising heater means for baking out said cryopumping means, said heater means being supported on said inner wall.

14. An ultrahigh vacuum pump for evacuating `an ultrahigh vacuum system comprising cryopumping means for removing condensable gases from said system, diffusion pumping means for removing non-condensable gases from said system, a single housing enclosing said cryopumping means and said diflusion pumping means, and heater means within said single housing for baking out said pump and said system.

15. In an ultrahigh vacuum pump for evacuating an ultrahigh vacuum system, a housing having a housing wall, a tank for cryogenic fluid, said tank being located within said housing and comprising an outer wall spaced from said housing wall to provide an insulating space therebetween, and an inner wall defining a gas passageway therethrough, heater means for baking out said pump, and means supporting said heater means on said inner wall in said gas passageway.

16. In an ultrahigh vacuum pump as recited in claim 15, said heater meanscomprising a heater wire helically wound on the inner surface of said inner wall.

17. In an lultrahigh vacuum pump as recited in claim 16, a first feed-through por-t extending through said housing wall, a second feed-through port aligned with said first feed-through por-t and extending through said tank between said insulating space and said gas passageway, sealing means covering said first feed-through port, and conductor means extending through said sealing means, said first port, and said second port and connected to said heater wire.

18. In an ultrahigh vacuum pump for evacuating an ultrahigh vacuum system, a housing having a housing wall, a tank for cryogenic fluid, said tank being located within said housing and comprising an outer wall spaced from said housing wall to provide an insulating space therebetween, and an inner wall defining a gas passageway therethrough, a first feed-through port extending through said housing wall, a second feed-through port aligned with said first feed-through port and extending through said tank between said insulating space and said gas passageway, and means for sealing said first feed-through port.

19. In an ultrahigh vacuum pump, cyropumping means comprising a tank for a cryogenic fluid, said tank having an inner wall defining a gas passageway therethrough, ballie means in said passageway, means supporting said baille means on said inner wall, baille means comprising a baille plate mounted within said gas passageway, but spaced 'from said inner wall, and means conducting said cryogenic fluid to said baille plate.

20. In an ultrahigh vacuum pump as recited in claim 19, said means conducting said cryogenic fluid to said baille plate comprising a pipe extending transversely across said ypassageway between openings through said inner wall, said baille plate bing mounted on said pipe.

21. In an ultrahigh vacuum pump as recited in claim 20, said tank being substantially toroidal, said pipe extending diametrically across said passageway, said baille plate comprising a substantially semi-circular disc mounted midway of said pipe on one side thereof and extending in a plane normal to said passageway, and a second substantially semi-circular bafiie plate mounted midway of said pipe on the other side thereof and extending in said plane.

22. In an ultrahigh vacuum pump, cyropiumping means comprising a tank for a cryogenic fiuid, said tank having an inner wall defining a gas passageway therethrough, baffle means in said gas passageway, and means supporting said baflie means on said inner wall, said baffle means comprising a baflie flange mounted on said inner wall and extending inwardly therefrom and having a central opening therethrou-gh, and said baliie means further comprising a baffle plate mounted within said gas passageway, but spaced from said inner wall, and means conducting said cryogenic liuid to said baflie plate.

23. In an ultrahigh vacuum pump, cyropumping means comprising a tank for a cryogenic fluid, said tank having an inner wall defining a gas passageway therethrough, baflie means in said gas passageway, and means supporting said baie means on said inner wall, said baffle means comprising a bafiie flange mounted on said inner wall and extending inwardly therefrom and having a central opening therethrough, and said baffle means further comprising a second bafe flange mounted on said inner wall and extending inwardly therefrom and having a central opening therethrough, said second baffle flange being spaced from said first-men-tioned bathe flange, and a heater wire wound on said inner wall between said bafiie fianges.

24. In an ultrahigh vacuum pump, cryopumping means comprising a substantially toroidal tank having an inner wall defining a gas passageway, heater means for baking out said cryopumping means, and means mounting said heater means on said inner wall within said gas passageway.

25. In a system of `the character described, a tank having a top wall and a bottom wall; means for filling said tank with cryogenic fluid in a liquid phase comprising a fill pipe extending upwardly through said bottom wall to a first point adjacent, but below, said top wall, said fill pipe having a U-bend at said first point and extending downwardly to a second point adjacent, but above, said bottom wall, said fill pipe having a discharge opening at said second point; and means for exhausting said fluid in a gaseous phase from said tank comprising an exhaust pipe extending upwardly through said bottom wall to a third point adjacent, but below, said top wall, said exhaust pipe having an exhaust opening at said third point, said fill pipe and said exhaust pipe 4constituting the only openings -to said tank and said tank being filled with said cryogenic tiuid in said liquid phase to a level just below said first and third points and the upwardly extending portion of said fill pipe and said U-bend being filled with said fluid in said gaseous phase to form a gas lock.

26. In a system as recited in claim 25, said fluid being nitrogen.

27. In a system of the character described, a tank having a top wall and a bot-tom wall; means for filling said tank with a cryogenic fluid in a liquid phase comprising a fill pipe, said fill pipe having a portion below the bottom wall and a portion extending upwardly through the bottom wall of said tank to a first point adjacen-t, but below, said top wall, said fill pipe having a U-bend at said first point and extending downwardly to a second point adjacent, but above, said bottom wall, said fill pipe having a ydischarge opening at said second point, said tank being filled with said cryogenic fluid in said liquid phase to a level just below said iirst point; said portion of said fill pipe below the wall being exposed to ambient conditions, whereby upon closing said fill pipe after said tank is filled to said level the cryogenic fluid in said liquid phase in said portion will evaporate and cause said cryogenic fluid in said liquid phase in said upwardly extending portion and said U-bend to be blown around said bend to form a gas .lock in said upwardly extending portion and said U-bend.

28. In a diffusion pump having a boiler for producing a pumping vapor, a jet stack assembly comprising means mounting a baseplate above said boiler, means mounting an outer upwardly extending jet pipe on said baseplate, means mounting a rst jet cap at the upper end of said first jet pipe, said lirst jet cap having a central opening and flange means extending into said central opening and defining upper and lower shoulders, a first inner jet pipe section mounted on said baseplate and extending upwardly into said central opening and abutting said lower shoulder, a second inner jet pipe section mounted on said upper shoulder and extending upwardly therefrom, the outer diameter of said inner pipe sections being substantially equal to the diameter of said central opening, means mounting a second jet cap on the upper end of said second inner jet pipe section, and a tie rod extending between said second jet cap and said baseplate, whereby said jet stack assembly is self-aligning, said base plate having openings permitting said pumping vapor to travel upwardly through said outer jet pipe to said jet cap and through said inner je-t pipe sections to said second jet cap.

29. In a diffusion pump as recited in claim 28, said means mounting said outer jet pipe comprising a second outer jet pipe of greater diameter than said first-mentioned outer jet pipe, said second outer jet pipe being mounted on said base plate and extending upwardly therefrom, a third jet cap mounted on the upper end of said second outer jet cap, said third jet cap having a central opening having a diameter substantially equal to the outer diameter of said rst-mentioned outer jet pipe, second ange means extending into said central opening of said third jet cap and defining a second upper shoulder, said firstmentioned jet pipe being mounted within said central opening of said third jet cap on said second upper shoulder.

3i). In a diffusion pump having a boiler for producing a pumping vapor, heater means mounted below said boiler, and quick cool means mounted below said heater means Ito rapidly cool said boiler upon shut-down of said pump, said quick cool means comprising a quick cool plate contiguous to said heating means and a cooling coil contiguous to and below said quick cool plate.

31. In a diffusion pump as recited in claim 30, means for clamping said quick cool plate against said heater means.

32. An ultraihi gh vacuum pump for evacuating an ultrahigh vacuum system to pressures below 10-8 torr, comprising in combination: a single, unitary pump barrel having an upper section and a lower section, said upper sec- -tion being of greater diameter than said lower section, and a shoulder interconnecting said upper and said lower sections; cryopumping means within said upper section for removing condensable gases from said system comprising a substantially toroidal tank for a cryogenic iiuid located above said shoulder, said tank having an inner wall defining a gas passageway, spaced batiie flanges extending inwardly from said inner wall into said passageway, baflie plate means located substantially centrally of said passageway and between said lianges, a pipe extending substantially diametrically across said passageway between openings through said inner wall and supporting said baffle plate means, and a heater wire wound on said inner wall between said flanges; and diffusion pumping means for removing non-condensable gases from said system within said lower section.

33. An ultrahigh vacuum pump as recited in claim 32, said tank having a cylindrical outer wall spaced from said upper barrel section, and said inner wall having a diameter substantially equal to the diameter of said lower barrel section.

34. An ultrahigh vacuum pump as recited in claim 32, further comprising means extending through said shoulder for filling said tank with said cryogenic liuid and means extending through said shoulder for draining said cryogenic fluid from said tank.

35. In a diffusion pump having a boiler for producing a pumping vapor, means for heating pumping oil within said boiler, and anti-bumping means Within said boiler, 2,286,207 said anti-bumping means comprising a plurality of su- 2,703,673 perposed anti-bumping grids each having a plurality of 2,855,140 cells, adjacent grids being relatively staggered with the 3,168,819 cells of one grid being out of alignment with the cells of 5 an adjacent grid.

References Cited by the Examiner UNITED STATES PATENTS 2,282,777 5/ 1942 Gunderson 122-459 6/1942 Keenan et al 122-459 3/1955 Winkler 230-101 10/1958 Sedlacsik 230-101 2/1965 Santeler 230-101 X FOREIGN PATENTS 9/ 1960 Great Britain.

MARK NEWMAN, Primary Examiner.

8/ 1928 Pflug 222-464 X 10 WARREN E. COLEMAN, Examiner. 

1. AN ULTRAHIGH VACUUM PUMP FOR EVACUATING AN ULTRAHIHG VACUUM SYSTEM COMPRISING CRYOPUMPING MEANS FOR REMOVING CONDENSABLE GASES FROM SAID SYSTEM, DIFFUSION PUMPING MEANS FOR REMOVING NON-CONDENSABLE GASES FROM SAID SYSTEM, A SINGLE INTEGRAL HOUSING ENCLOSING SAID CYROPUMPING MEANS AND SAID DIFFUSION PUMPING MEANS, SAID HOUSING HAVING AN OPEN END AND A SUBSTANTIALLY CLOSED END, SAID CRYOPUMPING MEANS BEING LOCATED BETWEEN SAID DIFFUSION PUMPING MEANS AND SAID OPEN END, AND DETACHABLE MEANS INCLUDING A FILL PIPE AND AN EXHAUST PIPE FOR SAID CRYOPUMPING MEANS FOR SUPPORTING SAID CRYOPUMPING MEANS IN SAID HOUSING SO THAT SAID CRYOPUMPING MEANS MAY BE READILY REMOVED FROM SAID HOUSING THROUGH SAID OPEN END TO PROVIDE ACESS TO SAID DIFFUSION PUMPING MEANS FOR SERVICING. 